This invention relates to weighing scales and more particularly to a combination hydraulic-pneumatic weighing scale intended primarily for medical office and home bathroom use. As those skilled in the art are aware many types of bathroom platform scales are available over a wide range of prices. There are several types available including the mechanical fulcrum and lever types, spring type units and either air actuated or hydraulic types. Most of the scales, however, and particularly those that are of the low priced spring and lever type, involve a considerable amount of error. They contain too many parts, have too much built-in friction and they cannot be calibrated accurately except at zero weight. Purely hydraulic units have tried to do the job with one small type of load cell which requires a high pressure gauge. Thus the problem with purely hydrauic units is the high operating pressure. Additionally, they require evacuation of all air in the system to prevent bottoming of the load cell before full travel of the indicator can be effected. Where a weighting scale system involves high pressure it is susceptible to developing leaks and hence further inaccuracies. Also, single load cell weighing scales have small fluid displacement so that calibrating the indicator needle accurately to the scale is difficult if not impossible. In short, a small amount of liquid moved means high pressure and a small amount of deflection in the platform and load cell for difficult gauge calibration. Another problem is that most present bathroom scales do not distribute the load equally. Accuracy may depend upon proportional centering of the load on the platform and such equalizing of the load is not always achieved.
Patents which are of interest but not particularly pertinent to the instant invention are U.S. Pat. Nos. 2,516,545; 3,433,316; 3,637,034 and 3,765,497.